1. Field of the Invention
This invention relates to electronic circuits. In particular, this invention relates to input/output circuits on integrated circuits. More particularly, this invention relates to a circuit for reducing overshoot and undershoot on high speed signals that are interconnected between several integrated circuits.
2. Background of the Invention
Multiple integrated circuit devices are often all connected to a common signal. One example of this is a data line on a microprocessor connected to multiple dynamic random access memory (DRAM) chips. To simplify the layout of the printed circuit board, the traces connecting all of these chips may be routed as a main trunk line with smaller "stubs" branching off of that trunk line to connect to each individual device. This is often called a bus. Although convenient, this arrangement can cause severe glitches and signal integrity problems. This is particularly true when the rise or fall time of the signal is less than the propagation time of the signal down a stub. Signal integrity problems arise when a fast rising or falling signal is reflected off the open circuit impedance of the end of a stub and propagated back down the stub onto the trunk line. These reflections dynamically add to, and subtract from, the desired voltage on the signal at all locations on the bus, possibly causing glitches to appear and increasing settling time.
Accordingly, there is a need in the art for an improved way of reducing reflections on bus stubs. Such a way should not involve slowing the rise or fall time of the signal because that would slow the overall speed of the bus down. Furthermore, such a way should not reduce the voltage swing on the bus because this reduces the noise immunity of the bus. These and other needs are accomplished by the present invention.